N-acetyl-para-aminophenol (APAP) is a well-known analgesic which has enjoyed tremendous commercial success. There are various processes for manufacturing the same, including acylating para-aminophenol with an acetylating agent such as acetic anhydride.
Recently, a novel process for the production of APAP has been discovered and it is the subject matter of U.S. Pat. No. 4,524,217.
The above patent discloses the production of APAP by reacting 4-hydroxyacetophenone (4-HAP) with a hydroxylamine salt and excess base to form the ketoxime of the ketone and subjecting the ketoxime to a Beckmann rearrangement in the presence of a catalyst to form the N-acyl-para-aminophenol (APAP).
As can well be appreciated, in a multi-step process such as that disclosed in U.S. Pat. No. 4,524,217, the entire disclosure of which is incorporated herein by reference, the rate and/or manner at which one intermediate is produced is not necessarily the same as for another.
Thus, in a proposed manufacturing operation for APAP, a continuous process is used for the manufacture of 4-hydroxyacetophenone whereas a batch process is used for the manufacture of the oxime from 4-HAP and the APAP from the oxime. Consequently, there is a delay time between the production of 4-HAP and its subsequent utilization to produce the oxime and the final product. A very convenient way of handling this situation is to provide a surge system which basically consists of a holding tank or tanks between the 4-HAP and the oxime/APAP production.
Although it is very possible to hold 4-HAP in a separate vessel, nevertheless there are problems which are associated with such a procedure.
It has been found that 4-HAP is thermally unstable at elevated temperatures and tends to degrade when held in a molten condition for prolonged periods of time. On the other hand, when molten 4-HAP is mixed with water at 60.degree. C. and a 4-HAP/water/oil is formed, this material solidifies at about 45.degree. C. Thus, in order to hold 4-HAP until it can be used, a problem has arisen in that lower temperatures result in solidification and higher temperatures cause thermal degradation. 4-HAP can be recovered as a solid but this also causes problems, namely, the problems involved in the solid handling of 4-HAP as well as the additional cost which is required to recover 4-HAP as a solid.